Lee Child, bestselling author of the Jack Reacher thrillers, offered these insights in a workshop which I attended:
A series character makes it easier for a reader to buy a book, and sometimes encourages them to buy previous books in the series.
You can’t design a series character to be succesful. Let the character be himself and hope for the best. Don’t worry whether the character will be liked or disliked.
Allow the main character to be a little rugged, a tad dastardly. Many writers use a sidekick to be the tough one so the main character can remain pure. The main character should have those “dark” elements. This can work well with female characters, too.
On character development, Child believes his readers are looking for the same character in different situations. Series characters don’t even have to age. Readers can always count on Jack Reacher. This works well for Jack Reacher, but not for all series characters, like Harry Potter, or Christa Devlin, the main character in The Seventh Stone, who are altered dramatically by the experiences.
But the most important tip Child offered: Be yourself. Close your eyes and jump. Don’t be intimidated.